Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, “ultra-low” temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations, and semiconductor manufacturing and test applications.
In many of these applications, it is necessary that a system element, such as a semiconductor wafer holder or other device [hereafter sometimes referred to as an external heat load heat exchanger] be cycled through both heating and cooling regimes, depending on the specific processing step. During normal operations, it is necessary to cool and maintain the device at ultra-low temperatures.
During start up, or when vacuum has been lost or the process interrupted for some reason, it is necessary to supply high heat. In the case of an external heat load heat exchanger, such as a semiconductor wafer chuck in a clean room environment, a bakeout process is needed to clean the external heat load heat exchanger by burning off any accumulated impurities. A bakeout process is the heating of all surfaces in a vacuum chamber to remove water vapor and other contaminants after the chamber has been exposed to the atmosphere, such as occurs when the chamber is opened for maintenance. Conventional techniques of performing a bakeout process involve heating the surfaces of the system element with a heater to above +200° C. for a prolonged period of time.
In these applications, a temperature modification system must also be capable of accommodating the bakeout process and the post bakeout cooling requirements of the system where the element must be brought down to or near ambient temperature prior to the commencement or resumption of normal operations. Consequently, it is necessary that the system provides a bakeout cucle, as well as a post-bake cooling cycle, different from the normal cooling cycle, in which the external heat load heat exchanger is cooled from the bakeout temperature down to near ambient temperature. Thereafter, the normal cooling cycle brings the element down to the normal cold operating temperature range between −50 and −150° C.
For the purposes of this application, “heating” refers to the addition of heat from an object or fluid, “refrigeration” refers to the removal of heat from an object or fluid (gas or liquid) at temperatures below room temperature, and “ultra-low” temperature refers to the temperature range between −50 and −150° C.
For the purposes of this application, a heat exchanger means a device that causes heat to be transferred from one media to another.
All heat exchangers described in this application are indirect heat exchangers, that is, the media do not come into physical contact.
An external heat load heat exchanger refers to the thermal interface at which heat is removed from an object or fluid and transferred to a cooling medium.
Prior art gas systems have not been integrated systems and have not contemplated providing both heating and refrigeration within the same system. Furthermore, prior art chilling systems used to provide ultra-low temperature chilled gas to such applications are of open loop design.
Various refrigeration cycles may be utilized to provide the ultra-low temperatures for the chilled gas such as a Missimer type auto-refrigerating cascade, U.S. Pat. No. 3,768,273; a Klimenko type single-phase separator system, or a single expansion device type such as disclosed in U.S. Pat. No. 5,441,658. Further examples of open loop gas chillers are products made by IGC Polycold Systems (formerly of San Rafael, Calif., now located in Petaluma, Calif.), such as the PGC-150 and the PGC-100. Such systems typically are used to chill a stream of pressurized nitrogen gas from room temperature to between −90 C and −130° C., depending on the specific model and flow rate, with the flow rates of the cooled gas ranging between 0 and 15 scfm.
In current open loop systems, ambient temperature gas at low to medium pressure is chilled to ultra-low temperatures in an open loop where the chilled gas provides the necessary cooling to the external heat load heat exchanger or other surface to be cooled. After providing cooling to the external heat load heat exchanger the gas is vented. The refrigeration process has the benefit of being able to operate in steady-state conditions for extended time periods of days to months provided there is continuous supply of fresh, clean, and dry gas.
However, there are numerous negative aspects to such systems.
In open loop gas chilling systems the refrigerant gas is simply exhausted into the surrounding environment after the external heat load heat exchanger has been cooled. Consequently, a gas source must be provided that is capable of continuously replenishing the refrigerant gas within the refrigeration system in order to maintain proper gas pressure and flow rate. The need to provide a continuous gas supply is very costly to the user, is not cost-effective due to the open loop design and is a serious drawback of the prior art gas chilling systems.
Since in open-loop gas refrigeration systems the ultra-low temperature gas is simply exhausted into the surrounding environment after the external heat load heat exchanger has been cooled, there is a tendency for condensation and frost buildup to occur on the exhaust vent that is typically located within a semiconductor manufacturing clean room.
Consequently, another drawback of the prior art open-loop gas chilling systems is the detrimental presence of condensation and frost within a clean room environment of a semiconductor manufacturing process. Similarly, in the bakeout process, simply exhausting high temperature gas into the surrounding environment may be detrimental to the semiconductor manufacturing process and to the environment.
Lastly, in the case of a large-scale manufacturing process having multiple external heat load heat exchangers to be cooled, a very large gas flow rate is required to achieve the cooling of multiple external heat load heat exchangers. Since open-loop gas chilling systems require a gas source to continuously replenish the spent gas, a gas source capable of supplying this large volume of gas is needed in order to maintain the proper gas pressure and flow rate to all external heat load heat exchangers to be cooled.
Thus, another drawback of the prior art open loop gas chilling systems is the requirement of having a gas source capable of supplying the large volume of gas needed for the cooling of multiple external heat load heat exchangers.
Recently, refrigeration systems have appeared which are based on closed loop principles. For examples, U.S. Pat. No. 6,105,388, entitled “Multiple Circuit Cryogenic Liquefaction Of Industrial Gas”; U.S. Pat. No. 6,041,621, entitled “Single Circuit Cryogenic Liquefaction Of Industrial Gas”; and U.S. Pat. No. 6,301,923, entitled “Method For Generating A Cold Gas,” describe various methods of generating a closed loop gas stream cooled by a refrigeration system.
In semiconductor manufacturing processes refrigeration is required to reduce the temperature of the object being cooled from an initial temperature in the range of 250 to 300° C. such as a chuck used in processing semiconductor wafers, or any other such device. When closed loop refrigeration systems are utilized to cool the hot semiconductor objects, the extra heat load imposes a serious constraint on the process as it is necessary in such processes to deal with the hot gas that returns to the closed loop system as the initially very hot object cools
Since the systems described in U.S. Pat. Nos. 6,105,388, 6,041,621 and 6,301,923 are concerned with the production of industrial gases from an ambient temperature gas source, these systems are directed to and only address the basic refrigeration function. Such systems do not provide multicycle integrated temperature modification and are unable to handle heat exchange media returning from a hot external heat load heat exchanger.
Prior closed loop refrigeration processes do not recognize or resolve the problem of managing returning gas at high temperatures. Therefore, the arrangement of components described in the prior art would fail to function as needed for the processes just described.
Industrial processes intended for continuous running must address the potential for leaks to assure that the system may continually operate over long periods of time even during the occurrence of minor leaks that cause a loss of gas.
Thus, there is a need in the industry for a chilling process that is capable of cooling an initially hot object, that does not require the provision of large amounts of cooling fluid, that does not exhaust spent coolant fluid to the atmosphere and that provides for the replenishment of small quantities of make-up heat exchange medium as required.
It is therefore an object of the invention to provide a closed-loop gas chiller system, thereby providing a means to recirculate chilled gas and eliminate the costly need to constantly replenish the total flow of chilled gas supplied to the customer-installed external heat load heat exchanger, such as a chuck used in processing semiconductor wafers, or any such device
It is another object of the invention to use a gas rather than a liquid as a cooling medium in a closed-loop system.
It is yet another object of the invention to eliminate the exhaust vent that over time may accumulate a frost buildup.
It is yet another object of the invention to eliminate hot gas being exhausted into the manufacturing environment, such as a clean room, during the bakeout process.
It is yet another object of the invention to manage the post bakeout high temperature gas returning from the hot external heat load heat exchanger without adversely affecting the primary loop refrigeration system.
It is yet another object of the invention to eliminate the need for a large-capacity supply line for maintaining sufficient gas pressure and flow rate within a large-scale open-loop process, utilizing multiple customer-installed external heat load heat exchangers.
It is yet another object of the invention to have an automatic make up of circulating gas to replenish the gas lost in the system due to leaks, to maintain the desired operating pressures on the suction and discharge side of a secondary loop gas, to allow for contraction and expansion of gas due to variation in the gas temperature, and to provide a continuous operation.